This invention relates to the art of power supplies and, more particularly, to a controller for a series resonant power converter of the switching regulator type.
Power converters are known in the art and typically serve to accept energy from an unregulated energy source, such as a voltage source, and derive therefrom a regulated voltage which is applied to a load circuit. The regulation function is performed by interposing a regulating device between the source of energy and the load circuit.
One form of regulating device known in the prior art includes switching type regulating devices interposed between the source of energy and the load. These operate in a discontinuous manner in controlling the rate of energy transmission and, hence, consume less power during the regulating operation than regulating devices of the variable impedance type. The switching device has two modes of operation, fully on and fully off. The switching device is periodically turned on for a time interval to permit energy transfer for purposes of maintaining the power output at a predetermined level.
Typically, such switching type regulating devices employed in power converters utilize semiconductor devices, such as power transistors, as the switching devices. These devices are turned fully on, or saturated, or fully off during operation. When fully on, the semiconductor devices are conducting and little or no power is dissipated. Also, when nonconducting or fully off no power is dissipated therein. Power is, however, dissipated in such a semiconductor device during the time interval of switching from a nonconducting condition to a conducting condition and vice versa. It is during the switching time interval that a substantial amount of power may be dissipated in such a semiconductor device, and if large enough this may severely damage the semiconductor device.
It is desirable to provide power converters of the switching type which are cost effective and which occupy a small amount of space. This, then, necessitates high frequency operation, such as in excess of 20 KH.sub.z and preferably at substantially higher frequencies, such as 250 KH.sub.z, while transferring large amounts of power, such as on the order of 200 watts or more. In order to achieve such high frequency operation of power switches, sinusoidal operation in the form of a series resonant converter has been employed as opposed to the squarewave operation typically employed in the prior art.
One such example of a series resonant converter including a control circuit therefor is shown in U.S. Pat. No. 4,648,017 entitled "Control Of A Series Resonant Converter" which issued on Mar. 3, 1987 and is assigned to the same assignee as is the present invention. As described in U.S. Pat. No. 4,648,017, the control operates to ensure that the then on FET is turned off only when minimal or zero resonant current is flowing in the resonant circuit. The off FET is then turned on based on load requirements.
It is desirable to include in the control, circuit means which inhibits the false turning off of the then on FET. Noise in the resonant current may give rise to a false indication of zero or minimal resonant current to thereby cause the on FET to be prematurely turned off. Such premature turnoff endangers the operation of the FETs by causing them to switch current. This is not the intended mode of operation for the FETs.
It is also desirable to include means for obtaining a minimum deadtime between the turnoff of the on FET and the start of the sequence to develop a signal from the driver circuit to turnon the off FET and the actual generation of that signal.
It is further desirable to include means which provides for an orderly shutdown of the supply in the event of the occurrence of certain predetermined conditions which require a shutdown. This assures that the supply is shutdown only after the resonant current has become zero.
As described in U.S. Pat. No. 4,648,017 the FETs are turned on at a rate which depends upon load. The frequency of occurrence of the resonant current pulses becomes lower as the load becomes lighter. If the load is light enough the frequency of occurrence of the resonant current pulses becomes low enough to fall within the audible range of frequencies. That is undesirable. Therefore it is desirable to include in the supply, circuit means which limits the operating frequency at light loads such that it is not within the audible range of frequencies. One such example of such circuitry is shown in U.S. Pat. No. 4,587,604 entitled "Power Supply Employing Low Power and High Power Series Resonant Circuits" and which issued on May 6, 1986. That patent is assigned to the same assignee as is the present invention. The supply shown therein includes both low power and high power series resonant circuits. Both circuits operate when the load is heavy. The high power circuit is controlled to cease its operation when the load decreases to a certain level.